1. Field of the Invention
The present invention is directed to a protection circuit for protecting a load circuit having a power supply, a switch, and a load. The protection device detects an increase of temperature of a conductive wire used therein.
2. Description of the Related Art
A conventional load circuit supplying power to a load such as a bulb, a motor or the like, has a battery and an electric switch (semiconductor switch, etc.) provided between the battery and the load. These components are connected by conductive wires. The load circuit further has a control device to turn on/off the electric switch. Specifically, the control device outputs a drive signal or a stop signal to the electric switch so that the load is driven or stopped.
In the load circuit as described above, a protection function provided by a fuse or the like so as to shut down the power immediately when a current flowing in the load exceeds a predetermined threshold value of the current, so that damage to the load, the wire, the electric switch and the like is avoided.
The protection function as described above can protect the load circuit itself only in a dead short circuit, that is, when the current has apparently exceeds the threshold value. However, it cannot do so in a rare short circuit, which is a state where the current is larger than a normal value, but does not exceed the threshold value.
Under such a rare short circuit condition, temperature of the conductive wire would rise due to Joule heat generated therein. If a heating rate of the wire exceeds a cooling rate thereof, problems such as smoke emission from the wire or burnout of the wire may occur.
To solve such a problem, a protection apparatus is known in Japanese Patent Laid-Open Publication 2002-084654. According to this device, when a current flows in a load, the device calculates Joule heat based on the measured current. When the current does not flow, it calculates radiated heat, and further it calculates heat generated by arcing which occurs just after a power supply has been shut down. If the total heat calculated from the sum of the radiated heat and the generated heat exceeds a predetermined value, the device would shut down a circuit supplying power to the load.
However, the protection apparatus for the load circuit as disclosed above determines whether the load circuit is shut down or not depending on cumulative heat of generated and radiated heat and it does not take into account an effective rate of increase of temperature. That is, if a thick wire was used and the generated heat therefrom was large, temperature of the wire would not rise very much because the heat radiated from the wire sufficiently exceeds the heat generated therein. Consequently, a problem would occur, in which the circuit would be forcibly shut down irrespective of the fact that power can still be applied to the load device.
On the contrary, if a thin wire was used and the amount of generated heat was small, temperature of the wire would unexpectedly rise but the circuit would not be shut down irrespective of substantial smoke emission from the wire and burnout thereof.